Chapter 12
Support for
Object-Oriented
Programming
Introduction
• Many object-oriented programming (OOP)
languages
– Some support procedural and data-oriented
programming (e.g., Ada 95+ and C++)
– Some support functional program (e.g., CLOS)
– Newer languages do not support other
paradigms but use their imperative structures
(e.g., Java and C#)
– Some are pure OOP language (e.g., Smalltalk &
Ruby)
– Some functional languages support OOP, but
they are not discussed in this chapter
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Object-Oriented Programming
• Three major language features:
– Abstract data types (Chapter 11)
– Inheritance
• Inheritance is the central theme in OOP and
languages that support it
– Polymorphism
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Inheritance
• Productivity increases can come from reuse
– ADTs are difficult to reuse—always need
changes
– All ADTs are independent and at the same level
• Inheritance allows new classes defined in
terms of existing ones, i.e., by allowing
them to inherit common parts
• Inheritance addresses both of the above
concerns--reuse ADTs after minor changes
and define classes in a hierarchy
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Object-Oriented Concepts
• ADTs are usually called classes
• Class instances are called objects
• A class that inherits is a derived class or a
subclass
• The class from which another class inherits
is a parent class or superclass
• Subprograms that define operations on
objects are called methods
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Object-Oriented Concepts (continued)
• Calls to methods are called messages
• The entire collection of methods of an
object is called its message protocol or
message interface
• Messages have two parts--a method name
and the destination object
• In the simplest case, a class inherits all of
the entities of its parent
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Object-Oriented Concepts (continued)
• Inheritance can be complicated by access
controls to encapsulated entities
– A class can hide entities from its subclasses
– A class can hide entities from its clients
– A class can also hide entities for its clients while
allowing its subclasses to see them
• Besides inheriting methods as is, a class
can modify an inherited method
– The new one overrides the inherited one
– The method in the parent is overriden
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Object-Oriented Concepts
(continued)
• Three ways a class can differ from its
parent:
1. The parent class can define some of its
variables or methods to have private access,
which means they will not be visible in the
subclass
2. The subclass can add variables and/or methods
to those inherited from the parent
3. The subclass can modify the behavior of one or
more of its inherited methods.
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Object-Oriented Concepts (continued)
• There are two kinds of variables in a class:
– Class variables - one/class
– Instance variables - one/object
• There are two kinds of methods in a class:
– Class methods – accept messages to the class
– Instance methods – accept messages to objects
• Single vs. Multiple Inheritance
• One disadvantage of inheritance for reuse:
– Creates interdependencies among classes that
complicate maintenance
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Dynamic Binding
• A polymorphic variable can be defined in a
class that is able to reference (or point to)
objects of the class and objects of any of its
descendants
• When a class hierarchy includes classes that
override methods and such methods are
called through a polymorphic variable, the
binding to the correct method will be
dynamic
• Allows software systems to be more easily
extended during both development and
maintenance
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Dynamic Binding Concepts
• An abstract method is one that does not
include a definition (it only defines a
protocol)
• An abstract class is one that includes at
least one virtual method
• An abstract class cannot be instantiated
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Design Issues for OOP Languages
•
•
•
•
•
•
•
The Exclusivity of Objects
Are Subclasses Subtypes?
Single and Multiple Inheritance
Object Allocation and Deallocation
Dynamic and Static Binding
Nested Classes
Initialization of Objects
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The Exclusivity of Objects
• Everything is an object
– Advantage - elegance and purity
– Disadvantage - slow operations on simple objects
• Add objects to a complete typing system
– Advantage - fast operations on simple objects
– Disadvantage - results in a confusing type system (two
kinds of entities)
• Include an imperative-style typing system for
primitives but make everything else objects
– Advantage - fast operations on simple objects and a
relatively small typing system
– Disadvantage - still some confusion because of the two
type systems
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Are Subclasses Subtypes?
• Does an “is-a” relationship hold between a
parent class object and an object of the
subclass?
– If a derived class is-a parent class, then objects
of the derived class must behave the same as
the parent class object
• A derived class is a subtype if it has an is-a
relationship with its parent class
– Subclass can only add variables and methods
and override inherited methods in “compatible”
ways
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Single and Multiple Inheritance
• Multiple inheritance allows a new class to
inherit from two or more classes
• Disadvantages of multiple inheritance:
– Language and implementation complexity (in
part due to name collisions)
– Potential inefficiency - dynamic binding costs
more with multiple inheritance (but not much)
• Advantage:
– Sometimes it is quite convenient and valuable
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Allocation and DeAllocation of Objects
• From where are objects allocated?
– If they behave like the ADTs, they can be
allocated from anywhere
• Allocated from the run-time stack
• Explicitly create on the heap (via new)
– If they are all heap-dynamic, references can be
uniform thru a pointer or reference variable
• Simplifies assignment - dereferencing can be
implicit
– If objects are stack dynamic, there is a problem
with regard to subtypes – object slicing
• Is deallocation explicit or implicit?
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Dynamic and Static Binding
• Should all binding of messages to methods
be dynamic?
– If none are, you lose the advantages of dynamic
binding
– If all are, it is inefficient
• Maybe the design should allow the user to
specify
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Nested Classes
• If a new class is needed by only one class,
there is no reason to define so it can be
seen by other classes
– Can the new class be nested inside the class
that uses it?
– In some cases, the new class is nested inside a
subprogram rather than directly in another class
• Other issues:
– Which facilities of the nesting class should be
visible to the nested class and vice versa
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Initialization of Objects
• Are objects initialized to values when they
are created?
– Implicit or explicit initialization
• How are parent class members initialized
when a subclass object is created?
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Support for OOP in Smalltalk
• Smalltalk is a pure OOP language
– Everything is an object
– All objects have local memory
– All computation is through objects sending
messages to objects
– None of the appearances of imperative
languages
– All objected are allocated from the heap
– All deallocation is implicit
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Support for OOP in Smalltalk (continued)
• Inheritance
– A Smalltalk subclass inherits all of the instance
variables, instance methods, and class methods
of its superclass
– All subclasses are subtypes (nothing can be
hidden)
– All inheritance is implementation inheritance
– No multiple inheritance
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Support for OOP in Smalltalk (continued)
• Evaluation of Smalltalk
– The syntax of the language is simple and
regular
– Good example of power provided by a small
language
– Slow compared with conventional compiled
imperative languages
– Dynamic binding allows type errors to go
undetected until run time
– Introduced the graphical user interface
– Greatest impact: advancement of OOP
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Support for OOP in C++
• General Characteristics:
–
–
–
–
–
Evolved from C and SIMULA 67
Among the most widely used OOP languages
Mixed typing system
Constructors and destructors
Elaborate access controls to class entities
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Support for OOP in C++ (continued)
• Inheritance
– A class need not be the subclass of any class
– Access controls for members are
–
–
–
Private (visible only in the class and friends)
(disallows subclasses from being subtypes)
Public (visible in subclasses and clients)
Protected (visible in the class and in subclasses,
but not clients)
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Support for OOP in C++ (continued)
• In addition, the subclassing process can be
declared with access controls (private or
public), which define potential changes in
access by subclasses
– Private derivation - inherited public and
protected members are private in the subclasses
– Public derivation public and protected members
are also public and protected in subclasses
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Inheritance Example in C++
class base_class {
private:
int a;
float x;
protected:
int b;
float y;
public:
int c;
float z;
};
class subclass_1 : public base_class { … };
//
In this one, b and y are protected and
//
c and z are public
class
//
//
//
subclass_2 : private base_class { … };
In this one, b, y, c, and z are private,
and no derived class has access to any
member of base_class
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Reexportation in C++
• A member that is not accessible in a
subclass (because of private derivation) can
be declared to be visible there using the
scope resolution operator (::), e.g.,
class subclass_3 : private base_class {
base_class :: c;
…
}
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Reexportation (continued)
• One motivation for using private derivation
– A class provides members that must be visible,
so they are defined to be public members; a
derived class adds some new members, but
does not want its clients to see the members of
the parent class, even though they had to be
public in the parent class definition
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Support for OOP in C++ (continued)
• Multiple inheritance is supported
– If there are two inherited members with the
same name, they can both be referenced using
the scope resolution operator (::)
class
class
class
{ …
Thread { ... }
Drawing { ... }
DrawThread : public Thread, public Drawing
}
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Support for OOP in C++ (continued)
• Dynamic Binding
– A method can be defined to be virtual, which
means that they can be called through
polymorphic variables and dynamically bound to
messages
– A pure virtual function has no definition at all
– A class that has at least one pure virtual
function is an abstract class
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Support for OOP in C++ (continued)
class Shape {
public:
virtual void draw() = 0;
...
};
class Circle : public Shape {
public:
void draw() { ... }
...
};
class Rectangle : public Shape {
public:
void draw() { ... }
...
};
class Square : public Rectangle {
public:
void draw() { ... }
...
};
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Square* sq = new Square;
Rectangle* rect = new Rectangle;
Shape* ptr_shape;
ptr_shape = sq; // points to a Square
ptr_shape ->draw(); // Dynamically
// bound to draw in Square
rect->draw(); // Statically bound to
// draw in Rectangle
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Support for OOP in C++ (continued)
• If objects are allocated from the stack, it is
quite different
Square sq;
// Allocates a Square object from the stack
Rectangle rect; // Allocates a Rectangle object from the stack
rect = sq;
// Copies the data member values from sq object
rect.draw(); // Calls the draw from Rectangle
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Support for OOP in C++ (continued)
• Evaluation
– C++ provides extensive access controls (unlike
Smalltalk)
– C++ provides multiple inheritance
– In C++, the programmer must decide at design
time which methods will be statically bound and
which must be dynamically bound
• Static binding is faster!
– Smalltalk type checking is dynamic (flexible, but
somewhat unsafe)
– Because of interpretation and dynamic binding,
Smalltalk is ~10 times slower than C++
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Support for OOP in Java
• Because of its close relationship to C++, focus is
on the differences from that language
• General Characteristics
– All data are objects except the primitive types
– All primitive types have wrapper classes that store one
data value
– All objects are heap-dynamic, are referenced through
reference variables, and most are allocated with new
– A finalize method is implicitly called when the garbage
collector is about to reclaim the storage occupied by the
object
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Support for OOP in Java (continued)
• Inheritance
– Single inheritance supported only, but there is
an abstract class category that provides some of
the benefits of multiple inheritance (interface)
– An interface can include only method
declarations and named constants, e.g.,
public interface Comparable <T> {
public int comparedTo (T b);
}
– Methods can be final (cannot be overriden)
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Support for OOP in Java (continued)
• Dynamic Binding
– In Java, all messages are dynamically bound to
methods, unless the method is final (i.e., it
cannot be overriden, therefore dynamic binding
serves no purpose)
– Static binding is also used if the methods is
static or private both of which disallow
overriding
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Support for OOP in Java (continued)
• Evaluation
– Design decisions to support OOP are similar to
C++
– No support for procedural programming
– No parentless classes
– Dynamic binding is used as “normal” way to
bind method calls to method definitions
– Uses interfaces to provide a simple form of
support for multiple inheritance
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Support for OOP in C#
• General characteristics
– Support for OOP similar to Java
– Includes both classes and structs
– Classes are similar to Java’s classes
– structs are less powerful stack-dynamic
constructs (e.g., no inheritance)
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Support for OOP in C# (continued)
• Inheritance
– Uses the syntax of C++ for defining classes
– A method inherited from parent class can be
replaced in the derived class by marking its
definition with new
– The parent class version can still be called
explicitly with the prefix base:
base.Draw()
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Support for OOP in C#
• Dynamic binding
– To allow dynamic binding of method calls to
methods:
• The base class method is marked virtual
• The corresponding methods in derived classes are
marked override
– Abstract methods are marked abstract and must
be implemented in all subclasses
– All C# classes are ultimately derived from a
single root class, Object
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Support for OOP in C#
• Evaluation
– C# is a relatively recently designed C-based OO
language
– The differences between C#’s and Java’s support
for OOP are relatively minor
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Support for OOP in Ada 95+
• General Characteristics
– OOP was one of the most important extensions
to Ada 83
– Encapsulation container is a package that
defines a tagged type
– A tagged type is one in which every object
includes a tag to indicate during execution its
type (the tags are internal)
– Tagged types can be either private types or
records
– No constructors or destructors are implicitly
called
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Support for OOP in Ada 95 (continued)
• Inheritance
– Subclasses can be derived from tagged types
– New entities are added to the inherited entities
by placing them in a record definition
– All subclasses are subtypes
– No support for multiple inheritance
• A comparable effect can be achieved using generic
classes
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Example of a Tagged Type
package Person_Pkg is
type Person is tagged private;
procedure Display(P : in out Person);
private
type Person is tagged
record
Name : String(1..30);
Address : String(1..30);
Age : Integer;
end record;
end Person_Pkg;
with Person_Pkg; use Person_Pkg;
package Student_Pkg is
type Student is new Person with
record
Grade_Point_Average : Float;
Grade_Level : Integer;
end record;
procedure Display (St: in Student);
end Student_Pkg;
// Note: Display is being overridden from Person_Pkg
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Support for OOP in Ada 95 (continued)
• Dynamic Binding
– Dynamic binding is done using polymorphic
variables called classwide types
• For the tagged type Person, the classwide type is
Person‘ class
– Other bindings are static
– Any method may be dynamically bound
– Purely abstract base types can be defined in Ada
95 by including the reserved word abstract
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Support for OOP in Ada 95 (continued)
procedure Display_Any_Person(P: in Person) is
begin
Display(p);
end Display_Any_Person;
…
with Person_Pkg; use Person_Pkg;
with Student_Pkg; use Student_Pkg;
P : Person;
S : Student;
Pcw : Person’class; -- A classwide variable
…
Pcw := P;
Display_Any_Person(Pcw); -- Calls the Display in Person
Pcw := S;
Display_Any_Person(Pcw); -- Calls the Display in Student
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Support for OOP in Ada 95 (continued)
• Child Packages
– A child package is logically (possibly physically)
nested inside another package; if separate, they
are called child library packages
– Solves the problem of packages becoming
physically too large
– Even the private parts of the parent package are
visible to the child package
– A child package is an alternative to class
derivation
– A child library package can be added any time
to a program
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Support for OOP in Ada 95 (continued)
• Evaluation
– Ada offers complete support for OOP
– C++ offers better form of inheritance than Ada
– Ada includes no initialization of objects (e.g.,
constructors)
– Dynamic binding in C-based OOP languages is
restricted to pointers and/or references to
objects; Ada has no such restriction and is thus
more orthogonal
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Implementing OO Constructs
• Two interesting and challenging parts
– Storage structures for instance variables
– Dynamic binding of messages to methods
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Instance Data Storage
• Class instance records (CIRs) store the state
of an object
– Static (built at compile time)
• If a class has a parent, the subclass
instance variables are added to the parent
CIR
• Because CIR is static, access to all instance
variables is done as it is in records
– Efficient
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Dynamic Binding of Methods Calls
• Methods in a class that are statically bound
need not be involved in the CIR; methods
that will be dynamically bound must have
entries in the CIR
– Calls to dynamically bound methods can be
connected to the corresponding code thru a
pointer in the CIR
– The storage structure is sometimes called
virtual method tables (vtable)
– Method calls can be represented as offsets from
the beginning of the vtable
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Summary
• OO programming involves three fundamental concepts:
ADTs, inheritance, dynamic binding
• Major design issues: exclusivity of objects, subclasses and
subtypes, type checking and polymorphism, single and
multiple inheritance, dynamic binding, explicit and implicit
de-allocation of objects, and nested classes
• Smalltalk is a pure OOL
• C++ has two distinct type systems (hybrid)
• Java is not a hybrid language like C++; it supports only OOP
• C# is based on C++ and Java
• Ruby is a relatively recent pure OOP language; provides some
new ideas in support for OOP
• Implementing OOP involves some new data structures
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